This invention concerns a novel precipitated calcium carbonate of spherical morphology as well as a method for its preparation and its use in such as paper coatings.
Today's coated papers are graded primarily on the basis of their brightness. Within each grade, however, exist subcatgories defined by sheet gloss. Papers with a 75.degree. sheet gloss of from 1 to 20% are generally considered matte finish papers, from 20 to 40% dull finish paper, from 40 to 55% mid-gloss papers, and above 55% enamel papers. These definitions are general, with some overlap in sheet gloss and brightness between consecutive grades. Certain esthetic optical properties of paper are also commonly associated with functional performance. This is seen, for example, in the relationship of high sheet gloss to high print gloss and a high degree of printing smoothness. Dull and matte finish papers have historically been rougher and lower in print gloss than enamel papers, a consequence of the pigments and formulations utilized and the finishing techniques employed in their manufacture.
While numerous pigment combinations have been employed in attempting to produce a coated paper having a combination of dull finish and good printability, the need still exists for a coating pigment formulation which will accomplish this in a simple and economic manner. This need is satisfied by the present invention.
Precipitated calcium carbonate in spherical or globular form is known. In U.S. Pat. No. 3,304,154, for example, a finely divided precipitated calcium carbonate of generally spheroidal form is prepared by reacting a suspension of calcium hydroxide with carbon dioxide in a closed vessel at elevated temperature and pressure while rotating the vessel. In U.S. Pat. No. 3,848,059, spheroidal shaped reticulated precipitated calcium carbonate of 0.1 to 5 microns is prepared by the double decomposition of two water-soluble salts such as calcium chloride and potassium carbonate dissolved in water droplets of two water-in-oil emulsions. Nakahara et al, J. Chem. Soc. Japan, 5, 732 (1976), discloses the preparation of a globular or amorphous calcium carbonate by a similar interfacial reaction technique. Japanese Kokai No. 55-95617 discloses the preparation of globular-shaped calcium carbonate prepared by solution reaction of soluble calcium and carbonate salts at 70.degree. C. or below. Japanese Kokai No. 57-92520 purportedly prepares a spherical vaterite by a similar technique but in the presence of a divalent cation other than calcium, conversion of the vaterite to calcite being disclosed in Kokai 57-92521. Japanese Kokai No. 54-4300 prepares a fine spherical calcium carbonate by spraying a pressurized aqueous carbonate solution into an aqueous calcium salt solution, while Kamiya et al, Mat. Res. Bull., 12, 1095 (1977) discloses a calcium carbonate hydrate spherulite precipitated in a solution containing magnesium ion which decomposes in water to aragonite. Buehrer et al, J. Phys. Chem., 44, 552 (1940) discloses a distorted calcite in apparently globular form precipitated from solution in the presence of sodium hexametaphosphate. Such precipitation of calcium carbonate in the presence of sodium hexametaphosphate is also disclosed in U.S. Pat. Nos. 3,179,493, 4,018,877 and 4,244,933 as well as by Reitemeier et al, J. Phys. Chem., 44, 535 (1940). Of these precipitations, only U.S. Pat. Nos. 4,018,877 and 4,244,933 contemplate carbonating a lime slurry.
In U.S. Pat. No. 4,157,379, a corpuscular calcium carbonate is prepared by carbonation of a calcium hydroxide slurry in the presence of both a chelating agent and a water-soluble metal salt, including sodium pyrophosphate. Japanese Kokai No. 46-7374 discloses the production of large, crude crystals of precipitated calcium carbonate by reacting milk of lime and carbon dioxide following addition of condensed alkali phosphate, while Japanese Kokai No. 46-14821 discloses the addition of alkali polyphosphate plus an inorganic electrolyte to an already precipitated calcium carbonate to redisperse it in water.